When the brakes are applied on a vehicle traveling on a driving surface at a given velocity, a braking torque is generated at each of the braked wheels. Each braking torque causes a retarding or braking force to be generated at an interface between a corresponding tire and the driving surface. The braking force generated at each of the wheels then causes a decrease in the velocity of the vehicle.
Ideally, the braking force at each of the braked wheels would increase proportionately as the driver increases the force on the brake pedal. Unfortunately, this is not the case in all braking procedures. As the braking torque and hence the braking force at a wheel is increased, the rotational speed of the wheel becomes less than the speed of the vehicle. When the rotational speed of a wheel is less than the vehicle speed, "slippage" is said to occur between the tire and the driving surface. This slippage, when severe, can lead to lock-up of a wheel and skidding of the vehicle. In most cases, lock-up causes an increase in the stopping distance. Lock-up also causes a degradation in directional control due to a reduction in lateral forces at the wheels.
Both of the above-mentioned problems associated with lock-up were addressed with the introduction of anti-lock brake systems (ABS). A basic anti-lock brake system uses wheel speed sensors to monitor the velocity at one or more of the wheels, decides whether any of the wheels is at or approaching an excessive wheel slip condition based on these velocity measurements, and modulates a braking pressure accordingly to avoid lock-up. The ABS aids in retaining vehicle stability and steerability while providing shorter stopping distances.
An implementation of a general anti-lock brake system comprises one or more wheel speed sensors for sensing the rate of angular rotation of one or more selected wheels, one or more modulator units each capable of adjusting a brake actuating force to a corresponding wheel in response to an input signal, and a control unit which interprets input signals from the wheel speed sensors and transmits controlling output signals to the modulators. In a hydraulic brake system, each modulator contains one or more control valves for adjusting an application of hydraulic fluid pressure to a wheel cylinder capable of converting hydraulic fluid pressure to a mechanical force for actuation of a corresponding brake. To enable this function, each modulator is interposed between a brake master cylinder and a corresponding wheel cylinder.
The standard anti-lock brake system is capable of detecting excessive slip or wheel departure and wheel recovery from the slip based on input data. One method for identifying an excessive wheel slip condition in the control unit is by comparing the measured velocity of each wheel to a reference speed. The reference speed is an estimate of the true vehicle speed based on current and previous values of the individual wheel velocities. If the velocity of a wheel is less than the reference speed by a predetermined amount, then the wheel is deemed by the control unit to be excessively slipping. The control unit then commands a corresponding modulator to reduce the hydraulic pressure actuating a corresponding brake in order to reduce brake torque. The reduction of brake torque causes a reduction of the braking force, which then causes a reduction of the slip in the wheel.
In practice, the ABS reduces the brake pressure by first commanding a control valve to isolate brake fluid in an individual wheel brake from an increasing brake fluid pressure in the master cylinder. This isolation holds the brake fluid pressure in the wheel brake constant. Next, the ABS commands another control valve to bleed, or dump, brake fluid from the wheel brake to thereby reduce brake fluid pressure therewithin. Thereafter, the ABS typically holds brake fluid pressure in the wheel brake constant for an amount of time computed within the control unit. After a period of constant braking pressure following the pressure reduction, the pressure actuating the brake is increased until excessive wheel slip occurs again. The increase in brake pressure is accomplished by a reapplication of brake fluid to the wheel brake. The resulting cycle of decreasing the brake pressure, maintaining constant brake pressure, and then increasing brake pressure is repeated until excessive slip no longer occurs. The specifics of this brake pressure cycle depend on an anti-lock brake system algorithm that is employed within the control unit, along with both the vehicle and the driving surface conditions encountered during braking.